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Naming Of Aldehydes And Ketones

ChemistryAldehydes And KetonesFor NEET aspirants

Aldehydes and ketones both contain the carbonyl group, C=O, and this one group explains how they are named and how they react. This page covers the nomenclature and structure of carbonyl group compounds: common and IUPAC names of aldehydes and ketones, numbering rules, priority prefixes, isomerism, the sp2 orbital picture and the polarity of the C=O bond. The nomenclature and structure of carbonyl group questions in JEE Main, JEE Advanced and NEET build directly on these ideas.

Key facts: quick reference
  1. General formula (saturated, open-chain): CnH2nO, the same for aldehydes and ketones.
  2. Aldehyde R-CHO: suffix -al; the CHO carbon is always C-1.
  3. Ketone R-CO-R': suffix -one; give the carbonyl carbon the lowest locant.
  4. CHO on a ring: suffix -carbaldehyde (cyclohexanecarbaldehyde); benzaldehyde is also accepted.
  5. Lower priority than acid or ester: aldehyde or ketone oxygen becomes the prefix oxo-; CHO on a ring becomes formyl-.
  6. Common names: Greek letters start at the carbon next to C=O, so = C-2.
  7. Structure: carbonyl carbon is sp2, trigonal planar, bond angles about 120°; C=O is one and one bond.
  8. Polarity: ; carbon is electrophilic, oxygen is nucleophilic and basic.

1. Aldehydes, Ketones and the Carbonyl Group

Aldehydes and ketones are compounds containing a carbonyl group, C=O, in which a carbon atom is joined to an oxygen atom by a double bond. Saturated open-chain aldehydes and ketones have the same general formula, CnH2nO, so an aldehyde and a ketone with the same number of carbons are isomers.

  • Aldehyde, R-CHO: the carbonyl carbon carries at least one hydrogen. The -CHO unit is called the formyl group. R may be H, alkyl or aryl.
  • Ketone, R-CO-R': the carbonyl carbon is bonded to two carbon groups. R and R' may be the same (a simple or symmetrical ketone) or different (a mixed or unsymmetrical ketone).
  • Methanal (formaldehyde), HCHO: the simplest aldehyde. Its carbonyl group carries two hydrogen atoms and no alkyl group, so it is not surprising that it shows several characteristic properties of its own.
Aldehydes, ketones and methanal compared General structures. An aldehyde R-CHO has at least one hydrogen on the carbonyl carbon, and its CHO part is the formyl group. A ketone R-CO-R prime has two carbon groups on the carbonyl carbon, so the carbonyl group sits inside the chain. Methanal has two hydrogens and no alkyl group and often behaves differently. Saturated open-chain aldehydes and ketones share the formula CnH2nO. Aldehyde Ketone Methanal R H O R O R′ H H O formyl group, −CHO carbonyl group inside the chain carbonyl carbon has at least one H attached two carbon groups two H atoms, no alkyl group: often behaves differently Saturated open-chain aldehydes and ketones share the formula CnH2nO
Figure 1: General structures of aldehydes and ketones: the formyl group of an aldehyde, the carbonyl group inside the chain of a ketone, and methanal with two hydrogens.

2. Structure of the Carbonyl Group

Hybridisation and bonding

The carbonyl carbon uses three sp2 hybrid orbitals to form bonds with three atoms, one of which is oxygen. One unhybridised p orbital is left on carbon, perpendicular to the plane of the three bonds.

Oxygen forms a bond with carbon and carries two lone pairs. Oxygen is commonly described as sp2 hybridised, with both lone pairs in the plane of the molecule. Its remaining p orbital, holding one electron, is parallel to the p orbital on carbon. These two p orbitals overlap sideways to form the bond.

  • The carbonyl carbon, the oxygen and the two atoms attached to carbon all lie in one plane, with bond angles close to 120°. In methanal the H-C-H angle is about 116° and the H-C-O angles about 122°.
  • The C=O double bond consists of one bond and one bond. It is shorter (about 122 pm) and stronger than a C-O single bond (about 143 pm).
Orbital picture of the carbonyl group in methanal Orbital diagram of methanal. The carbonyl carbon is sp2 hybridised and forms three sigma bonds, to two hydrogens and to oxygen, lying in one plane about 120 degrees apart. The unhybridised p orbital on carbon overlaps sideways with a p orbital on oxygen above and below the plane to form the pi bond. Oxygen carries two lone pairs in the molecular plane. C O H H π bond: sideways overlap of p orbitals σ σ lone pairs on oxygen ~120° Carbon: sp2, three σ bonds in one plane, about 120° apart Unhybridised p orbital on C overlaps a p orbital on O: one π bond The C, O and the two atoms on C all lie in one plane
Figure 2: Orbital picture of the carbonyl group in methanal: sp2 carbon, three bonds in one plane, and a bond from sideways overlap of p orbitals.

Polarity of the carbonyl group

Oxygen is more electronegative than carbon, so the electron density is greater around oxygen than around carbon. The carbon and oxygen atoms therefore carry partial positive and partial negative charges, respectively. The polarity is shown by writing a dipolar resonance structure alongside the normal one:

This polarity gives carbonyl compounds sizeable dipole moments: methanal about 2.3 D, ethanal about 2.7 D, propanone about 2.9 D. It also decides their chemistry:

  • The carbon is electrophilic, so nucleophiles attack it. This is why aldehydes and ketones undergo nucleophilic addition.
  • The oxygen is nucleophilic and weakly basic. It is protonated by acids and bound by Lewis acids, which makes the carbon even more electrophilic.
Polarity of the carbonyl group and its reactive sites The carbonyl group is a resonance hybrid of the neutral form and a dipolar form with a positive carbon and a negative oxygen, so the real molecule has a partial positive carbon and a partial negative oxygen. As a result nucleophiles attack the carbonyl carbon, while protons and Lewis acids bind the oxygen. Two resonance structures describe the real carbonyl group R O R′ R O R′ main contributor explains the polarity R O R′ δ+ δ− real molecule What the polarity means for reactions R O R′ δ+ δ− Nu− nucleophiles attack carbon H+ H+ and Lewis acids bind oxygen
Figure 3: Resonance and polarity of the carbonyl group: the dipolar form explains the carbon attacked by nucleophiles and the oxygen that binds H+.
Why the carbonyl double bond reacts differently from an alkene double bond Comparison. The alkene double bond has the same atom at both ends, shares electrons evenly, and its pi electrons attract electrophiles, so alkenes undergo electrophilic addition. The carbonyl double bond joins carbon to the more electronegative oxygen, so electrons are pulled toward oxygen, the partially positive carbon attracts nucleophiles, and carbonyl compounds undergo nucleophilic addition. Alkene double bond Carbonyl double bond H H H H H H O δ+ δ− same atoms at both ends electrons shared evenly π electrons attract electrophiles (E+) electrophilic addition O more electronegative than C electrons pulled toward O δ+ carbon attracts nucleophiles (Nu−) nucleophilic addition
Figure 4: Structure of carbonyl group compared with an alkene: C=C undergoes electrophilic addition, C=O undergoes nucleophilic addition.
Solved Example 1
State the hybridisation of each carbon atom in (a) CH3CHO and (b) CH2=CH-CHO. In which molecule do all the atoms lie in one plane?
Solution:

(a) In ethanal, the CH3 carbon forms four bonds, so it is sp3. The CHO carbon forms three bonds and one bond, so it is sp2.

(b) In prop-2-enal, all three carbons form a double bond, so all three are sp2.

Every atom in prop-2-enal is attached to an sp2 centre, and the conjugated system needs parallel p orbitals. All atoms of prop-2-enal lie in one plane. In ethanal, the tetrahedral CH3 group puts two of its hydrogens outside the plane.

3. Common Names

Aldehydes

The common name of an aldehyde comes from the common name of the carboxylic acid with the same carbon skeleton. The ending -ic acid is replaced by -aldehyde.

Acid (common name)AldehydeCommon nameIUPAC name
formic acid, HCOOHHCHOformaldehydemethanal
acetic acid, CH3COOHCH3CHOacetaldehydeethanal
propionic acidCH3CH2CHOpropionaldehydepropanal
butyric acidCH3(CH2)2CHObutyraldehydebutanal
valeric acidCH3(CH2)3CHOvaleraldehydepentanal
benzoic acidC6H5CHObenzaldehydebenzaldehyde (benzenecarbaldehyde)

Substituent positions in common names are shown with Greek letters. The carbon next to the carbonyl group is , the next is , then , and so on. So C-2 of the IUPAC name corresponds to of the common name.

IUPAC numbers versus Greek letters in common names of aldehydes The chain of 3-bromobutanal is numbered 1 to 4 starting from the aldehyde carbon for the IUPAC name. In the common name beta-bromobutyraldehyde, Greek letters start from the carbon next to the aldehyde group, so carbon 2 is alpha, carbon 3 is beta and carbon 4 is gamma, and the aldehyde carbon has no Greek letter. H3C Br H O 1 2 3 4 α β γ IUPAC: number from C of CHO 3-bromobutanal Common: Greek letters from the carbon next to CHO β-bromobutyraldehyde C-2 = α, C-3 = β, C-4 = γ (the CHO carbon itself gets no Greek letter)
Figure 5: Nomenclature of aldehydes: IUPAC numbers start at the CHO carbon, while common names use Greek letters starting from the next carbon.

Ketones

The simplest ketone has the common name acetone. For most other aliphatic ketones, name the two groups attached to the carbonyl carbon in alphabetical order and add the word ketone: ethyl methyl ketone, diethyl ketone, diisopropyl ketone.

A ketone in which the carbonyl group is attached to a benzene ring is named as a -phenone. CH3COC6H5 is acetophenone, and C6H5COC6H5 is benzophenone.

4. IUPAC Names of Aldehydes and Ketones

Aldehydes

  1. Choose the parent chain. Take the longest chain that contains the -CHO carbon.
  2. Change the ending. Replace the final -e of the corresponding alkane by -al: propane gives propanal.
  3. Number from the CHO carbon. The carbonyl carbon is always C-1, so no locant is written for it (butanal, not butan-1-al).
  4. Add substituents with their numbers in alphabetical order: 3-methylpentanal, 2-methoxypropanal.
  5. Two CHO groups at the chain ends: keep the -e and add -dial, as in butanedial (OHC-CH2-CH2-CHO).
  6. CHO attached to a ring: name the ring and add -carbaldehyde. The CHO carbon is not counted in the ring name: cyclohexanecarbaldehyde, benzene-1,2-dicarbaldehyde.

Ketones

  1. Choose the parent chain. Take the longest chain that contains the carbonyl carbon.
  2. Change the ending. Replace the final -e of the alkane by -one.
  3. Number the chain so that the carbonyl carbon gets the lowest possible number, and write that number before -one: pentan-2-one.
  4. Diketones keep the -e: pentane-2,4-dione. Cyclic ketones are named as cycloalkanones: cyclohexanone.
Numbering the chain of aldehydes and ketones Top: 2-methylpentanal, 3-methylpentanal and 4-methylpentanal, each numbered from the aldehyde carbon as carbon 1. Bottom: the ketone is named 3-methylbutan-2-one by numbering from the end nearer the carbonyl group; numbering from the other end, giving 2-methylbutan-3-one, is wrong because the carbonyl locant must be as low as possible. Aldehydes: the CHO carbon is always C-1 (no locant needed) H3C CH3 H O 1 2 3 4 5 2-methylpentanal H3C CH3 H O 1 2 3 4 5 3-methylpentanal H3C CH3 H O 1 2 3 4 5 4-methylpentanal Ketones: give the carbonyl carbon the lowest number H3C CH3 CH3 O 1 2 3 4 3-methylbutan-2-one correct: carbonyl at C-2 H3C CH3 CH3 O 4 3 2 1 2-methylbutan-3-one wrong: carbonyl locant is higher
Figure 6: Numbering rules: the aldehyde carbon is always C-1, and the ketone carbonyl carbon gets the lowest possible number.
Solved Example 2
Give the IUPAC name and the common name of (a) (CH3)2CHCH2CHO and (b) CH3CH(OCH3)CHO.
Solution:

(a) The longest chain containing CHO has four carbons. Numbering from CHO puts the methyl group on C-3. IUPAC: 3-methylbutanal. C-3 is the carbon, so the common name is -methylbutyraldehyde (also isovaleraldehyde).

(b) The chain has three carbons with -OCH3 on C-2. IUPAC: 2-methoxypropanal. C-2 is , so the common name is -methoxypropionaldehyde.

Solved Example 3
Give the IUPAC name of (a) CH3COCH(CH3)CH2CH3 and (b) (CH3)2CHCOCH(CH3)2. Also give their common names.
Solution:

(a) The longest chain containing the carbonyl carbon has five carbons. Numbering from the left gives the carbonyl carbon C-2 and the methyl group C-3. IUPAC: 3-methylpentan-2-one. The groups on the carbonyl carbon are methyl and sec-butyl, so the common name is sec-butyl methyl ketone.

(b) The chain has five carbons with the carbonyl carbon in the middle and methyl groups on C-2 and C-4. IUPAC: 2,4-dimethylpentan-3-one. Common name: diisopropyl ketone.

Name gallery

The structures below are the ones most often asked. Learn the IUPAC and common names together.

Common and IUPAC names of simple aldehydes Eight aliphatic aldehydes with IUPAC and common names: methanal (formaldehyde), ethanal (acetaldehyde), propanal (propionaldehyde), butanal (butyraldehyde), 2-methylpropanal (isobutyraldehyde), 3-methylbutanal (isovaleraldehyde), pentanal (valeraldehyde) and prop-2-enal (acrolein). H H O methanal formaldehyde H H3C O ethanal acetaldehyde H3C H O propanal propionaldehyde H3C H O butanal butyraldehyde H3C CH3 H O 2-methylpropanal isobutyraldehyde H3C CH3 H O 3-methylbutanal isovaleraldehyde H3C H O pentanal valeraldehyde H2C H O prop-2-enal acrolein
Figure 7: IUPAC and common names of aliphatic aldehydes, from methanal to prop-2-enal.
Names of aromatic and ring aldehydes Six aldehydes with rings: benzaldehyde (IUPAC also benzenecarbaldehyde), 4-nitrobenzaldehyde (p-nitrobenzaldehyde), 4-methylbenzaldehyde (p-tolualdehyde), 2-phenylethanal (phenylacetaldehyde), cyclohexanecarbaldehyde, and benzene-1,2-dicarbaldehyde (phthalaldehyde). H O benzaldehyde benzenecarbaldehyde H O NO2 4-nitrobenzaldehyde p-nitrobenzaldehyde H O CH3 4-methylbenzaldehyde p-tolualdehyde H O 2-phenylethanal phenylacetaldehyde H O cyclohexanecarbaldehyde H O H O benzene-1,2-dicarbaldehyde phthalaldehyde
Figure 8: Names of aromatic and ring aldehydes, including the carbaldehyde suffix.
Common and IUPAC names of simple ketones Nine ketones with IUPAC and common names: propanone (acetone), butan-2-one (ethyl methyl ketone), pentan-2-one (methyl n-propyl ketone), pentan-3-one (diethyl ketone), 3-methylbutan-2-one (isopropyl methyl ketone), 2,4-dimethylpentan-3-one (diisopropyl ketone), cyclohexanone, 2-methylcyclohexanone (alpha-methylcyclohexanone) and 4-methylpent-3-en-2-one (mesityl oxide). H3C CH3 O propanone acetone (dimethyl ketone) H3C CH3 O butan-2-one ethyl methyl ketone H3C CH3 O pentan-2-one methyl n-propyl ketone H3C O CH3 pentan-3-one diethyl ketone H3C CH3 CH3 O 3-methylbutan-2-one isopropyl methyl ketone H3C CH3 O CH3 CH3 2,4-dimethylpentan-3-one diisopropyl ketone O cyclohexanone O H3C 2-methylcyclohexanone α-methylcyclohexanone H3C O CH3 CH3 4-methylpent-3-en-2-one mesityl oxide
Figure 9: IUPAC and common names of aliphatic and cyclic ketones.
Names of aromatic ketones Six aromatic ketones: 1-phenylethanone (acetophenone), 1-phenylpropan-2-one (phenylacetone), 1-phenylbutan-1-one (butyrophenone), diphenylmethanone (benzophenone), 1-(2-hydroxyphenyl)ethanone (o-hydroxyacetophenone), and (4-methylphenyl)(4-nitrophenyl)methanone. H3C O 1-phenylethanone acetophenone CH3 O 1-phenylpropan-2-one phenylacetone H3C O 1-phenylbutan-1-one butyrophenone O diphenylmethanone benzophenone H3C O OH 1-(2-hydroxyphenyl)ethanone o-hydroxyacetophenone H3C O NO2 (4-methylphenyl)(4-nitrophenyl)methanone
Figure 10: IUPAC and common names of aromatic ketones such as acetophenone and benzophenone.

When a higher-priority group is present

Only one group can be the suffix of an IUPAC name. The order of priority for the suffix is carboxylic acid > ester > aldehyde > ketone > alcohol > amine. When a higher-priority group is present, the carbonyl oxygen is shown as the prefix oxo-. An aldehyde group attached to a ring is shown as the prefix formyl-.

Naming aldehydes and ketones when a higher-priority group is present Priority for the suffix is carboxylic acid, then ester, then aldehyde, then ketone, then alcohol. Examples: 4-oxobutanoic acid, where the aldehyde becomes the prefix oxo; ethyl 3-oxobutanoate, where the ketone becomes oxo; 3-oxobutanal, where the aldehyde takes the suffix and the ketone is oxo; and 4-formylbenzoic acid, where an aldehyde on a ring becomes the prefix formyl. Suffix priority: acid > ester > aldehyde > ketone > alcohol H O O OH 4-oxobutanoic acid aldehyde becomes the prefix oxo H3C O O O CH3 ethyl 3-oxobutanoate ketone as the prefix oxo H3C O H O 3-oxobutanal aldehyde beats ketone H O O OH 4-formylbenzoic acid aldehyde on a ring: formyl
Figure 11: Naming carbonyl compounds when a higher-priority group is present: the prefixes oxo and formyl.
Solved Example 4
Give IUPAC names: (a) OHC-CH2-COOH; (b) CH3COCH2CH2OH; (c) OHC-CH2-CH2-CH2-CHO; (d) CH3COCH2COCH3.
Solution:

(a) The acid is the suffix and the aldehyde oxygen is shown as oxo on C-3. 3-Oxopropanoic acid.

(b) The ketone outranks the alcohol, and numbering gives the carbonyl carbon C-2 and -OH C-4. 4-Hydroxybutan-2-one.

(c) There are two terminal CHO groups on a five-carbon chain. Pentanedial.

(d) There are two ketone groups on C-2 and C-4, and the -e is kept. Pentane-2,4-dione.

Solved Example 5
Give IUPAC names: (a) CH3CH=CHCHO; (b) (CH3)2C=CHCOCH3; (c) C5H9CHO (CHO on cyclopentane); (d) m-BrC6H4CHO.
Solution:

(a) Numbering from CHO puts the double bond at C-2. But-2-enal (crotonaldehyde).

(b) Numbering from the end nearer the carbonyl group gives C=O at C-2 and C=C at C-3, with a methyl group on C-4. 4-Methylpent-3-en-2-one (mesityl oxide).

(c) The CHO is attached to a ring. Cyclopentanecarbaldehyde.

(d) Bromine is on C-3 relative to the CHO carbon of the ring. 3-Bromobenzaldehyde (3-bromobenzenecarbaldehyde).

Solved Example 6
Draw the structures of (a) 4-chloropentan-2-one, (b) 3-oxopentanal and (c) 2-hydroxycyclopentanecarbaldehyde.
Solution:

(a) A five-carbon chain with C=O at C-2 and Cl at C-4: CH3COCH2CH(Cl)CH3.

(b) An aldehyde as C-1 and a ketone oxygen on C-3: OHC-CH2-CO-CH2-CH3.

(c) A cyclopentane ring carrying CHO on one carbon (C-1) and -OH on the next ring carbon (C-2): HO-C5H8-CHO, with the two groups on adjacent ring carbons.

Solved Example 7
Each name below is wrong. Give the correct IUPAC name: (a) 2-methylbutan-3-one; (b) butan-1-al; (c) 2-ethylpropanal; (d) pentan-4-one.
Solution:

(a) The carbonyl carbon must get the lower number. 3-Methylbutan-2-one.

(b) The aldehyde carbon is always C-1 and is not numbered. Butanal.

(c) An ethyl group on C-2 means the longest chain was missed: CH3CH2CH(CH3)CHO. 2-Methylbutanal.

(d) Numbering from the other end gives the carbonyl carbon C-2. Pentan-2-one.

5. Isomerism in Aldehydes and Ketones

  • Functional isomerism: an aldehyde and a ketone with the same formula, such as propanal and propanone (C3H6O).
  • Chain isomerism: different carbon skeletons, such as butanal and 2-methylpropanal.
  • Position isomerism (ketones): pentan-2-one and pentan-3-one differ in the position of the carbonyl group.
  • Metamerism (ketones): different alkyl groups on either side of the carbonyl group, such as methyl propyl ketone and diethyl ketone.
  • CnH2nO also fits unsaturated alcohols, unsaturated ethers and cyclic alcohols and ethers. Enols (C=C-OH) are tautomers of carbonyl compounds.
All aldehyde and ketone isomers of C5H10O Seven carbonyl compounds have the formula C5H10O. Four aldehydes: pentanal, 2-methylbutanal, 3-methylbutanal and 2,2-dimethylpropanal. Three ketones: pentan-2-one (methyl propyl ketone), pentan-3-one (diethyl ketone) and 3-methylbutan-2-one (isopropyl methyl ketone). Each aldehyde is a functional isomer of each ketone. Four aldehydes: chain and position isomers H3C H O pentanal H3C H3C H O 2-methylbutanal H3C CH3 H O 3-methylbutanal (CH3)3C H O 2,2-dimethylpropanal Three ketones: position isomers, chain isomers and metamers H3C CH3 O pentan-2-one methyl propyl ketone H3C O CH3 pentan-3-one diethyl ketone H3C CH3 CH3 O 3-methylbutan-2-one isopropyl methyl ketone Each aldehyde is a functional isomer of each ketone: 7 carbonyl isomers in all
Figure 12: All seven aldehyde and ketone isomers of C5H10O.
Solved Example 8
How many aldehydes and ketones have the formula (a) C4H8O and (b) C5H10O? Name them.
Solution:

(a) C4H8O has 3: butanal and 2-methylpropanal (aldehydes), and butan-2-one (ketone).

(b) C5H10O has 7. The aldehydes are pentanal, 2-methylbutanal, 3-methylbutanal and 2,2-dimethylpropanal. The ketones are pentan-2-one, pentan-3-one and 3-methylbutan-2-one.

Pentan-2-one and pentan-3-one are position isomers and also metamers, and 3-methylbutan-2-one is their chain isomer. 2-Methylbutanal has a chiral carbon, so counting stereoisomers would add one more.

Solved Example 9
Which of the following is a mixed ketone? (A) pentan-3-one (B) benzophenone (C) acetophenone (D) all of these
Solution:

A mixed (unsymmetrical) ketone has two different groups on the carbonyl carbon. Pentan-3-one has two ethyl groups and benzophenone has two phenyl groups, so both are simple ketones. Acetophenone, CH3COC6H5, has one methyl and one phenyl group.

Answer: (C) acetophenone.

Common Mistakes to Avoid

Watch out
  • Numbering a ketone from the wrong end. The carbonyl carbon gets the lowest locant, even if a substituent then gets a higher number: 3-methylbutan-2-one, not 2-methylbutan-3-one.
  • Writing a locant for the aldehyde carbon. It is always C-1: butanal, not butan-1-al.
  • Mixing Greek letters and numbers. is C-2, not C-1: 2-chloropropanal is -chloropropionaldehyde.
  • Counting the CHO carbon in a ring name. C6H11CHO is cyclohexanecarbaldehyde, not cycloheptanal.
  • Using -al or -one when an acid or ester is present. Use oxo- or formyl- instead: 4-oxobutanoic acid.
  • Choosing a longer chain that skips the carbonyl carbon. The parent chain must contain it: CH3CH2CH(CH3)CHO is 2-methylbutanal.
  • Calling the carbonyl carbon sp3 or the group non-planar. It is sp2 and trigonal planar, with angles near 120°.
  • Calling the C=O bond non-polar like C=C. Oxygen pulls electron density, making carbon and oxygen .

Frequently Asked Questions

What is the general formula of aldehydes and ketones?

Saturated open-chain aldehydes and ketones share the general formula CnH2nO. For aldehydes n starts at 1 (methanal, HCHO); for ketones n starts at 3 (propanone). Because they share a formula, an aldehyde and a ketone with the same carbon count are functional isomers, such as propanal and propanone.

How are aldehydes named in the IUPAC system?

Choose the longest chain containing the CHO carbon, replace the -e of the alkane with -al, and number from the CHO carbon, which is always C-1 and needs no locant. When CHO is attached to a ring, add -carbaldehyde to the ring name, as in cyclohexanecarbaldehyde.

How are ketones named in the IUPAC system?

Choose the longest chain containing the carbonyl carbon, replace the -e of the alkane with -one, and number the chain so the carbonyl carbon gets the lowest locant, as in pentan-2-one. Common names list the two attached groups followed by ketone, such as ethyl methyl ketone.

What is the hybridisation of carbon in the carbonyl group?

The carbonyl carbon is sp2 hybridised. It forms three sigma bonds in one plane at about 120° and uses its unhybridised p orbital to form a pi bond with oxygen. Oxygen is usually described as sp2 hybridised too, with two lone pairs in the plane.

Why is the carbonyl group polar?

Oxygen is more electronegative than carbon, so the pi electrons are pulled toward oxygen. Carbon becomes partially positive and oxygen partially negative, shown by the resonance form with C+ and O−. That is why nucleophiles attack the carbonyl carbon.

What do alpha and beta mean in the common names of aldehydes?

In common names, Greek letters mark positions starting from the carbon next to the carbonyl group. That carbon is alpha, the next is beta, then gamma. So alpha corresponds to C-2 in the IUPAC name: 3-bromobutanal is beta-bromobutyraldehyde.

What nomenclature questions on aldehydes and ketones appear in JEE Main and JEE Advanced?

JEE asks for correct IUPAC names of polyfunctional compounds, using priority rules with oxo and formyl prefixes, and for counting structural isomers of formulas like C5H10O. It also tests the sp2 geometry and polarity of the carbonyl group, which explain nucleophilic addition.

Which names of aldehydes and ketones should NEET students memorise?

NEET follows NCERT. Know the common and IUPAC names of formaldehyde, acetaldehyde, propionaldehyde, butyraldehyde, isobutyraldehyde, acrolein, benzaldehyde, acetone, ethyl methyl ketone, acetophenone, benzophenone and mesityl oxide, plus the -carbaldehyde and oxo rules and the sp2 structure of the carbonyl group.

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